Tridemensional Structures for an Ink Jet Printhead and Relevant Manufacturing

ABSTRACT

In a monolithic ink jet printhead, a structural layer is made comprising cavities, obtained fron the polymerization of a solution of a monomer, or an oligomer, and a photointiator; during the polymerization by radiation, acid species are unduly generated in zones protected by a mask, due to reflection of the radiation on reflecting surfaces of the support of the sructural layer; these acid species cause undue polymerization of the solution on the inside of the cavities to oppose the formation of these acid species, a polymerzation inhibitor basic compound is used.

BACKGROUND OF THE INVENTION

This invention relates to tridimensional structures, used in an ink jetprinthead and the relevant manufacturing process.

In particular the printhead that this invention relates to, is of thetype in which ink droplets are ejected through an ejection duct, ornozzle, communicating with a vaporisation chamber, in which a bubble ofvapour is produced.

BRIEF DESCRIPTION OF THE STATE OF THE ART

Ink jet printheads are known in the art, in which the energy forexpelling the drops of ink is obtained from the vaporisation of a smallvolume of ink contained in the vaporisation chambers, which is expelledthrough the nozzles communicating with these chambers.

Vaporisation of the ink is caused by a resistor placed within thechambers and suitably energized by means of electronic microcircuits.The ink is supplied through a feeding duct which is in communicationwith both the chamber, and also with a slot, made in the thickness of asilicon die.

The term “microhydraulics” is understood to mean as a whole the nozzles,the vaporisation chambers and the ink feeding ducts connected to them.

Similarly the term “microelectronics” is understood to mean as a wholethe active and passive electronic components, produced by means ofphotolithography on the silicon die.

Ink jet heads, called monolithic, have their microhydraulics made in asingle layer of material, called the structural layer. This can be doneby making templates of the hydraulic microcircuit with a sacrificiallayer, depositing a solution of photosensitive material on it,polymerizing it and finally removing the sacrificial layer.

The structure of a monolithic head is illustrated in FIG. 1 by means ofan axonometric projection sectioned parallel to the x-z axis; depictedin the figure are:

-   -   The silicon die 4, on which the head itself is built;    -   The structural layer 25;    -   The vaporisation chambers, or simply chambers, 26, made in the        structural layer 25; the figure shows only one of the chambers        26, in cross-section;    -   The nozzles 24, one of which shown in cross-section, made on the        top of the chambers 26;    -   The slot 27 and the ducts 22, made in the die 4 and shown in        cross-section in the figure, which convey the ink to the        chambers 26;    -   Electric contacts or “pads” 23;    -   apertures 12 in correspondence with the pads 23, made in the        structural layer.

The microelectronics are built on the upper face of the silicon die 4,and are not visible in FIG. 1 as they are hidden by the structural layer25.

The pads 23 have the function of electrically connecting themicroelectronics with the circuits external to the head, through contactwith special external conductors called “fingers”. The fingers and themicroelectronics are not depicted in any figures as they are notessential for an understanding of the invention.

Ink jet heads having for example 300 nozzles, a definition for exampleof 600 dots per inch (dpi), and suitable for emitting drops of volume 5pl for example, have chambers with dimensions in the order of tens of μm(for instance, they have side of about 20 μm and thickness about 10 μm),with tolerances in the order of a μm.

The apertures 12, leaving the pads uncovered, allow these to come intoelectrical contact with the fingers. The apertures have dimensions inthe order of tens or hundreds of μm (for instance, they have side ofabout 100 μm with tolerances in the order of a μm) and are typically ofthe same height as the chambers.

The microelectronics underneath are, on the other hand, made fromelements whose dimensions are in the order of a μm, meaning thereforethat they are typically smaller than the elements of the microhydraulicsby two orders of magnitude.

During production of the various layers of the microelectronics, layersof polymerizable materials are used for the purpose of selectivelyprotecting certain portions of the die being worked on, intended to beremoved later. These layers are substantially different from thestructural layer that makes up the microhydraulics, in particular theytypically have a thickness in the order of a μm or less and, as they areintended to be removed in later work phases, are made of materials oflow mechanical characteristics and adhesion to the adjacent layers.

To guarantee high reliability of the head, it is recommended that thestructural layer 25 displays excellent chemical inertness in respect ofthe ink, for instance even when the ink, for reasons linked to thestability of the raw materials used to manufacture it, has highalkalinity values. The structural layer should conveniently also have asufficient mechanical resistance and optimal adhesion to the substrate,even if remaining constantly in contact with the ink.

The U.S. Pat. No. 6,123,863 describes a process for the production of anink jet printhead comprising the phases of producing, in a sacrificiallayer of positive resist deposited on the silicon substrate, thetemplates of the hydraulic microcircuits, covering the sacrificial layerwith a structural layer made, inter alia, from a preparation based onphotopolymerizable epoxy resin cycloaliphatic (EHPE 3150 Daicel), andafter polymerization of the structural layer, removing the sacrificiallayer with one or more solvents belonging to the family of aliphaticesters of lactic acid chosen from methyl lactate, ethyl lactate andbutyl lactate.

In the context of the technology for forming ultrafine structures in themanufacture of semiconductors, U.S. Pat. No. 6,753,128 proposes aphotosensitive solution made up of a photopolymer, a photoinitiator anda basic additive suitable for neutralizing the acid species that havemigrated, or spread into unexposed areas.

The basic additive is made up of basic compounds with a fluorendiamineand naftalendiamine structure, with high degree of basicity, as thecoefficient of acidity of the conjugate acid (pKa) is between 12 and 16.

The basic additive is present in the photopolymerizable solution in anamount that is between 0.5% and 20% in weight of the photoinitiator. Theresults give a value for LER (Line Edge Roughness) of 9-13 nm, incomparison with a LER value of 10-20 nm obtained in the absence of theproposed additive.

U.S. Pat. No. 5,683,856 describes a process intended to improve thedefinition of ultrafine structures made in a polymerized layer, byeliminating the imperfections caused by a migration of acid species intoareas not exposed to radiation, or by basic impurities present in theatmosphere, which alter the concentration of the acid species on thesurface of the photopolymer. It proposes a photosensitive compositionwith positive action comprising a photoinitiator, a component inhibitingdissolution of the polymer and a basic species made up of polymerscontaining nitrogen. The results refer to a resolution, in thestructures made in this way, of between 260 and 480 nm, indicated asdifficult to obtain without the additive proposed.

U.S. Pat. No. 5,981,139 refers to the production of structures in aphotopolymer, comprising a photosensitive compound of the chemicalamplification type, containing a generator of photosensitive acidspecies; when this photopolymer is used in the presence of basicenvironmental contaminants, such as for instance, ammoniacal gases, orin contact with an isolating layer of silicon nitride, impregnated withammonia or water, or again in contact with layers of a phosphorous andboron based glass, deactivation of part of the acid species may beobserved. As a result, irregularly-shaped structures are formed, forinstance with rounded corners, or with polymer residues.

To avoid these imperfections the patent quoted proposes mixing a specialtype of aliphatic amine, or one of its salts, with the photopolymer, ina quantity less than 1% in weight.

The European patent application EP 1 253 138 relates to the problem ofeliminating, as in those quoted previously, the geometric deformationsof tridimensional ultrafine structures having resolution of 200 nm orless, caused by the presence of basic contaminant components present inthe air, which neutralize the acid species of the photopolymer; for thispurpose, the above-mentioned European patent application EP 1 253 138proposes using, as additives to the chemical amplification photopolymer,basic compounds consisting of tertiary amines containing an ester group.

The applicant observes that the above-mentioned patents relate to thetechnology of integrated circuits having patterns in the order of amicron, and tackle the problem of eliminating irregularities typicallyless than a micron (LER between 10 and 20 nm, for example).

Conversely, the structures of the microhydraulics relative to thisinvention have typical dimensions of tens or hundreds of μm, withtolerances in the order of the μm. These are not therefore affected bythe problems tackled by the above-mentioned patents, as the tolerancesare wider than the defects that these patents propose eliminating.

SUMMARY DESCRIPTION OF THE INVENTION

In connection with this invention the inventors have observed in anyevent that, though a preparation based on a cycloaliphatic epoxy resinpermits good adhesion to the substrate to be obtained on account of thelow volume shrinkage and has good characteristics of chemical inertnessin relation to the ink, important residues still remain of materialundesirably polymerized during the photolithographic process employed tomake the tridimensional structures, in particular during the chemicaletching process intended to selectively remove the masked and thereforeunpolymerized areas of the layer. These residues are indicated with thenumeral 40 in the photo of FIG. 2, obtained by means of an electron scanmicroscope.

Details are provided below of the composition and preparation of thepolymerizable mix, together with the conditions under which thephotolithographic process is conducted, in which the specimen of thephoto of FIG. 2 was obtained, with reference to the description ofexample no. 1.

The photo depicts a cavity 20, suitable for constituting, for instance,an aperture 12, and having dimensions of several hundreds of μm, asshown from the dimensional scale included in the picture. On the bottomwall 15 of the cavity is a metallic part 36 also having dimensions inthe order of hundreds of μm, suitable for constituting, for instance, apad or an electric conductor. The residues 40 have dimensions between,for instance, a few tens and a few hundreds of a μm, they thereforeconcern a large part of the area of the cavity 20 and of the metallicpart 36, and generate drawbacks in the subsequent head assemblyprocesses, drastically reducing the yields and increasing the productioncost: if for example, the metallic part 36 were to be a pad, theresidues 40 could prevent an effective electrical contact from beingestablished with the relative finger, meaning that the head in questionwould be rejected.

Not wanting to be bound to one theory, the inventors believe that thephenomenon of unwanted polymerization observed in the areas not exposedto radiation is due to the generation of acid species due to reflectionof the radiation on reflecting portions of the bottom wall of thetridimensional structures in the areas underneath the mask, and not tomigration of the acid species generated during the exposure. Thisunwanted polymerization extends over tens or even hundreds of μm, asalready observed in the photo of FIG. 2.

In the documents quoted earlier, the problem of diffusion of the acidspecies in the technology of ultrafine integrated circuit structuresconcerns dimensions of the order of a tens of nanometers, i.e. about twoorders of magnitude less than the desired tolerances in microhydraulicsof the heads, and from three to four orders of magnitude less than thedimensions of the residues observed by the inventors.

In this description, the term “acid species” is taken to mean hydrogenions or groups likely to promote the polymerization of cationicpolymerization monomers or oligomers.

In view of this, according to this invention it was found that it waspossible to obtain tridimensional structures in a polymerizable materialby photochemical means, free of residues even in the presence ofsubstrates at least in part reflecting, provided that effectiveopposition is provided to the undesired polymerization of thephotosensitive mix due to the generation of acid species in the areasnot exposed to radiation.

In particular, the inventors believed that this phenomenon of undesiredpolymerization was caused by the reflection of the radiation strikingsurfaces of any shape and extent, and typically takes place when thesurfaces reflect a percentage of incident radiation between 50 and 95%,and more specifically at least equal to 70%.

In the context of this invention the applicant has therefore noted thatthe problem of the presence of defects in the walls of the structurallayers in heads for ink jet printing, was due to reflectivity of thesubstrate on which said structural layer was made, and which couldtherefore be resolved by controlling the polymerization in the areasprotected by masking, in particular through the use of a controlledquantity of the polymerization inhibitor in said structural layer, saidquantity being insufficient to prevent polymerization in the areassubjected to direct radiation, but sufficient to ward off polymerizationin the areas exposed to reflection.

For simplicity's sake, in the description that follows, the termsreflect, reflection and derivatives are used also with the meaning ofdiffuse, diffusion and derivatives.

In particular, the method of producing tridimensional structures for anink jet printhead, according to the invention, makes use of a polymericstructural layer obtained from a photosensitive mix containing at leastone monomer or one epoxy oligomer, a photoinitiator capable ofgenerating Lewis acids when struck by UV radiation, and a basicpolymerization inhibitor compound, mixed in a measure at leastsufficient to neutralize the acid species unduly generated in the areasnot exposed to direct radiation, on account of reflection of theradiation on reflecting portions of the support surface.

At the same time, the inhibitor compound must be mixed in sufficientlylow measure to be fully consumed in the areas exposed to directradiation, and thus allow polymerization of the structural layer inthese areas.

In a first aspect, this invention relates to a photolithographic processfor making tridimensional structures in a structural layer inphotopolymer for an ink jet printhead. The tridimensional structures aremade on a substrate having a bottom wall at least in part reflecting,and comprise at least one side wall adjacent to said bottom wall. Theprocess comprises the operations of:

-   -   depositing on the substrate a photopolymerizable layer        comprising a solution of a monomer or oligomer, a        photoinitiator, and a polymerization inhibitor basic compound in        a quantity less than the stechiometric value corresponding to        complete neutralization of the acid species generated by the        photoinitiator;    -   masking the photopolymerizable layer to define areas protected        and areas not protected by the masking, intended to produce the        tridimensional structures;    -   irradiating with ultraviolet radiation (UV) to generate acid        species suitable for promoting polymerization of the solution in        the areas not protected by the masking, while a fraction of the        radiation is reflected by the reflecting part on the inside of        the areas protected by the masking and generates unwanted acid        species;    -   neutralizing the unwanted acid species by means of the inhibitor        basic compound;    -   removing the unpolymerized part of the photopolymerizable        solution from the areas protected by the masking.

Preferably, the polymerization inhibitor is chosen from:N,N-dimethylaniline; piridine; hydroxylamina; 2,6-dimethylpiridine;4-dimethylamminopiridine; imidazole; morpholine; hydrazine; piperidine;triisopropanolammina; methylamine; ethylamine; diethylamine;triethylamine.

Alternatively, the polymerization inhibitor comprisestris-(2-hydroxy-1-propyl)ammina, or piperidine, or4-dimethylamminopiridine.

Preferably, the polymerization inhibitor has a coefficient of acidity ofthe conjugate acid (pKa) of between 5 and 11, and is present in saidpolymerizable solution with a concentration between 0.05% and 0.4% byweight, with respect to the weight of the polymerizable solution,excluding the solvent.

Preferably, the stechiometric ratio of the polymerization inhibitor withrespect to the photoinitiator is between 1:10 and 9:10, and morepreferably between 1:8 and 3:8.

In particular, the reflecting part of the bottom wall of the substrateis made of a track of Aluminium.

Preferably, the tridimensional structures comprise at least one cavityof side at least 20 μm, having a bottom wall that comprises a reflectingportion, and the tridimensional structures have a thickness of at least5 μm.

In a second aspect, this invention relates to an ink jet printheadproduced by means of the process described above.

These and other characteristics of the invention will appear moreclearly from the following description of a preferred embodiment,provided by way of non-restrictive example, with reference to theaccompanying drawings..

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents in schematic form a monolithic ink jet printhead, seenin an axonometric projection and sectioned parallel to the x-z plane;

FIG. 2 is a microphotograph of a tridimensional structure made accordingto a method known in the prior art;

FIGS. 3, 4, 5 and 6 represent in schematic form how the inhibitionaction is carried out according to the invention;

FIG. 7 is a microphotograph of a tridimensional structure made accordingto a method known in the prior art;

FIGS. 8, 9, 10 and 11 are microphotographs of tridimensional structuresproduced according to different examples of the process of thisinvention.

DESCRIPTION OF A PREFERRED EMBODIMENT

The inventors noted that the phenomenon whereby a certain amount of acidspecies is present unwanted in the areas not exposed to polymerizingradiation, occurs significantly in the production of hollowtridimensional structures when the substrate on which these structureshave to be made is reflecting type, where by reflecting surface is meanta surface made of a material that reflects roughly between 50% and 95%(or more) of the incident radiation and more specifically at least about70% of said incident radiation.

The inventors saw that the reflection of the radiation on reflectingportions of the bottom wall of the tridimensional structures produces apolymerization of the photopolymerizable solution, including in areasprotected by the mask, independently of the presence of migration ofacid species, or of the presence of external impurities in thephotopolymerizable solution employed. It was in fact noted that theunwanted polymerization does not occur when the bottom wall hasreflectance less than approximately 50%, for example in the areas inwhich the structural layer is deposited directly on top of a substrateof silicon carbide, without further metallic layers.

The silicon, which has low reflectivity, is considered a nonmetal forthe purposes of this description.

According to this invention, in order to avoid the above-describedunwanted polymerization, a polymerization inhibitor basic additive ismixed with the photopolymerizable solution used in production of thestructural layer.

The process of producing tridimensional structures, according to theinvention, and the method of acting of the polymerization inhibitorbasic additive is described with reference to the FIGS. 3-6.

FIGS. 3-5 represent, schematically and not to scale, the die 4comprising at least one reflecting portion 5; on the die 4 a layer 6 isdeposited of a photopolymerizable solution containing a polymerizationinhibitor basic additive.

A mask 7, opaque to the radiation 8, covers a zone 9 and leavesuncovered surrounding zones 10. The layer 6 comprises B-basic species,consisting of a polymerization inhibitor basic additive, added to thepolymerizable solution of the layer 6.

During a first part of the exposure, H+ acid species are generated inthe layer 6 in the zones 10 exposed to the radiation 8; these acidspecies are quickly neutralized by the basic inhibitor, so thatpolymerization does not start; however, the inhibitor present in theexposed zones 10 is rapidly depleted and at a certain point (FIG. 4) allthe inhibitor will have disappeared, whereas in the zones 9 not exposed,under the mask 7, it is still present and active.

As the radiation continues, polymerization starts in the exposed zones10, with the generation of new acid species.

Through the effect of the reflections on the reflecting surface 5 on thechip 4 (FIG. 5), acid species are also generated under the mask 7, inthe zones not exposed 9, but are immediately neutralized by theinhibitor still present in the zone under the mask, and therefore noform of polymerization can start up in the zone 9.

When, in the successive phases of the process not depicted forsimplicity's sake, the layer 6 is developed with appropriate solvents,the zone 9 of the layer 6 which was under the mask 7, as it is notpolymerized, is totally removed.

A structural layer 25 is thus made, in the present case comprising acavity 20 (FIG. 6), which is bounded by substantially flat lateral walls14 (or at any rate with rectilinear generatrices), arranged parallel tothe direction of the radiation 8, and joined at a right angle to thebottom wall 15 of the chip 4 with sharp edges 16.

A photopolymerizable solution, according to the invention, consists ofan epoxy monomer or oligomer, preferably cycloaliphatic type, such asfor example 3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexane carboxylate(of the type CYRACURE UVR-6105, UV-6107 and UV-6110—DOW CHEMICAL Co),bis-(3,4-epoxycyclohexylmethyl) adipate (such as Cyracure UVR 6128—DowChemical Co.) and the product obtained from condensation of1,2-epoxy-4-(2-ossiranil)cyclohexane with2,2′-bis(hydroxymethyl)-1-butanol better known by its commercial name ofEHPE-3150 (Daicel Chemical Industries); a cationic photoinitiator, i.e.capable of generating acid species when irradiated with appropriateradiation. Among the cationic photoinitiators that may be used for thepurpose, there are the onium salts, such as arylsolfonium salts andaryliodonium salts, where the counter-ion may be thehexafluoroantimonate anion or the hexafluorophosphate anion.

Particularly preferred for its high level of reactivity is the mix ofbis(4-(diphenylsolfonium)phenyl)sulphide bis(hexafluoroantimonate) anddiphenyl((phenylthio)phenyl)solfonium hexafluoroantimonate, better knownby its commercial name of Cyracure UVI-6976. In addition to the epoxycompound and the cationic photoinitiator, the following may also bepresent in the mix: reticulating additives, to increase the percentageof conversion of the epoxy rings and make the layer obtained morechemically inert, such as aliphatic, cycloaliphatic and aromatic diolesand trioles, of the trimethylolpropane type, ε-caprolactone-triole,1,4-bis(2-hydroxyhexafluoropropyl)benzene; wetting agents to enhanceuniformity of application of the photosensitive mix on the substrate,such as silicon additives, of the type Dow Corning 57 (Dow Corning Co.);an adhesion promoter, to improve the adherence of the structural layerto the inorganic substrate, chosen in function of the type of inorganicsubstrate being used.

In particular, for a silicon substrate, or a compound of silicon such asfor example silicon carbide, oxide or nitride, or of metals that reacteasily with oxygen, adhesion promoters belonging to the family ofsilanes are particularly recommended, and more specifically for thepurposes of this invention, compounds belonging to the family offunctionalized epoxy silanes, such as (3-glicidossi)-trimetoxyisilane.

The photopolymerizable mix may also contain an organic solvent, toregulate viscosity and facilitate deposition on the substrate, throughapplication processes such as spray application, metallic bladeapplication, and spinner application (preferred in the photoresisttechnique).

The solvent is chosen from among those compatible with the components,the mix and in function of the chosen application process. Inparticular, for application with a spinner, the solvent must preferablyhave a vapour pressure such as to avoid it evaporating too quickly atambient temperature: particularly recommended for the purpose are thesolvents with a vapour pressure at 20° C. of less than 20 kPa andpreferably of less than 5 kPa.

According to the invention, for the purpose of avoiding unwantedpolymerizations of non-irradiated areas present on reflectingsubstrates, to the photopolymerizable mix described above a basicadditive, polymerization inhibitor is added, preferably chosen fromamong N,N-dimethylaniline; piridine; hydroxylamina;2,6-dimethylpiridine; imidazole; tris-(2-hydroxy-1-propyl)amine;hydrazine; 4-dimethylamminopiridine; morpholine; methylamine;ethylamine; diethylamine; triethylamine; piperidine.

Conveniently, the basic additive should respect a number of conditions,in order to obtain the best results in manufacturing tridimensionalstructures, free of geometrical irregularities; in particular:

a) the basic additive should be substantially soluble in the compositiondescribed, and in general, in organic solvents if present;

b) the coefficient of acidity of the conjugate acid (pKa) pf the basicadditive should preferably be between 5 and 11;

c) the basic additive should be provided with a low mobility in thepolymerizable solution, to avoid it from migrating, in the course of thepolymerization process, from the zones not exposed to radiation to thosethat are exposed;

d) the quantity of basic additive included in the photopolymerizablesolution must be stechiometrically less than that of the photoinitiator,in such a way that the additive is sufficient to neutralize the acidspecies generated by the reflected radiation in the zones that are notexposed, but is such as to allow in any case acid species to survive inthe exposed zones sufficient to promote polymerization.

In particular, the stechiometric ratio of the additive with respect tothe photoinitiator is preferably between 1:20 and 19:20 and morepreferably between 1:10 and 9:10.

In addition, the concentration by weight of the additive with respect tothe weight of the solution, solvent excluded, is preferably between0.05% and 0.4%.

More in general, the organic bases, such as the primary amines, thesecondary amines and preferably the tertiary amines have been found tobe suitable for use in accordance with the invention, as polymerizationinhibitor additives, unlike the inorganic bases, such metallichydroxides and ammonia.

EXAMPLE 1

This example has been produced without the addition of the basicadditive, and is given here for comparison with the examples from 3 to6, in which the basic additive was added.

In a glass container, 81 p/p of the oligomer cycloaliphatic epoxideproduced from the condensation of 1,2-epoxy-4-(2-ossiranil)-cyclohexaneand 2,2′-bis(hydroxymethyl)-1-butanol, 3.5 p/p of the cationicphotoinitiator an equimolar mix betweenbis(4-(diphenylsolfonium)phenyl)sulphide, bis(hexafluoroantimonate) anddiphenyl((phenylthio)phenyl)solfonium hexafluoroantimonate, and 15.5 p/pof the chain transfer agent 1,4-bis(2-hydroxyesafluoropropyl)benzenewere mixed in bis(2-methoxyethyl)ether.

Also added to the mix were 8 p/p of 3-glycidoxy-trimetoxysilane and0.002 p/p of the Dow Corning siliconic additive no. 57 (Dow CorningCo.).

Mixing of the components was produced by rotation in a trundler for 24h.

The mix described above was applied to a substrate of silicon carbidethat had a zone provided with a metallic coating, consisting of a layerof aluminium, by means of spin coating for 20 seconds at 2000 revs/min,giving a layer 25 μm thick.

The substrate with the mix deposited was radiated with UV radiationproduced by a mercury vapour lamp, with a suitable mask placed inbetween to define a pattern including a cavity. A total energy of 2.5J/cm² was radiated on the specimen.

The reflectivity of the substrate in the areas of the metallic coatingswas estimated at roughly 70-80%, whereas the reflectivity of thesubstrate in the zones without metallic coatings was evaluated at lessthan 40%.

At the end of the exposure, the non polymerized parts were removed byimmersion for 3 minutes in a 1/1 p/p mix of xylene andmethylisobutylchetone.

The definition of the pattern obtained and the presence or otherwise ofundesired residues was evaluated by observation under an electronicscanning microscope (SEM).

The product obtained from the mix and with the procedure described abovedemonstrated unacceptable definition characteristics and had amplepolymeric residues in the areas covered by the mask during the exposure,as illustrated in the microphotograph of FIG. 2.

On observing the figure, local to the reflecting metallic part 36 theside wall 14 was seen to have an undesired deposit 40, indicatingexcessive polymerization caused by the acid species unduly generated bythe reflection of the radiation, and not inhibited due to a lack of thespecific additive.

EXAMPLE 2

Again this example was produced without addition of the basic additive.

The composition and preparation of the mix were identical to thosedefined in example 1, but the substrate with the mix deposited on it inthis case was radiated with less energy than in example 1.

UV radiation produced by a mercury vapour lamp was used once again, witha suitable mask in between defining a pattern including two cavities,but this time radiating a total energy of 0.8 J/cm²on the specimen.

At the end of the exposure, the non-polymerized parts were removed byimmersion for 3 minutes in a 1/1 p/p mix of xylene andmethylisobutylchetone.

Definition of the pattern obtained and presence or otherwise ofundesired residues was evaluated by observation under an electronicscanning microscope (SEM).

The product obtained from the mix and with the procedure described abovestill demonstrated unacceptable definition characteristics and still hadpolymeric residues in the areas covered by the mask during the exposure,as illustrated in the microphotograph of FIG. 7.

FIG. 7 shows a cavity 20-1 with two separate metallic coatings 5 a and 5b (consisting of aluminium), adjacent to a structural layer 25 boundedby a side wall 14; as the figure shows, the metallic coating 5 b extendsunder the structural layer 25, while the metallic coating 5 a stops afew tens of microns from the wall 14 bounding the structural layer 25.

On observing the figure, local to the metallic coating 5 b, whichcontinues under the polymeric layer 25, the side wall 14 is seen to havean undesired deposit 40, indicating excessive polymerization. On theleft of the FIG. 7, on the other hand, it may be seen that the wall 14remains straight in correspondence with the metallic coating 5 a.

It is considered that the excess of polymerization observed was causedby the acid species unduly generated by the reflection of the radiationof the reflecting surface of the metallic coating 5 b, and not inhibiteddue to the lack of the specific additive, whereas this phenomenon didnot occur in the adjacent portion of the polymeric layer, in theproximity of the metallic coating 5 a, since the latter, as it did notcontinue under the exposed area, could not perform reflection. Thisdemonstrates clearly that, under the experimental conditions describedabove, excessive polymerization occurred which extended for a few tensof μm in the areas concerned by reflection of the radiation whereas, inthose areas where reflection did not occur, no visible presences werenoted of polymerized residual material not removed by the development.

This leads us to believe that the excessive polymerization observed isnot due to causes such as, for example, migration or diffusion of acidspecies into areas not exposed to radiation, or the presence ofcontaminants presents in the environment, which, in any case couldinvolve, at most, thicknesses in the order of a few tens of nanometers,dimensions that cannot be observed on the scale of the elements inquestion.

EXAMPLE 3

To the mix obtained following the previous example a polymerizationinhibitor basic additive was added, consisting oftris-(2-hydroxy-1-propyl)ammina in a stechiometric ratio of 1:3.8 to thephotoinitiator.

The mix thus prepared was used under the same conditions as the processdescribed in example 1, in which the radiated energy was suitablyincreased to take account of the lesser reactivity of thephotopolymerizable layer. The value taken was of 3.5 J/cm².

In this case, the results observed under the SEM, reproduced in FIG. 8,were excellent. No residues were observed in the masked areas and theside walls of the cavities were substantially vertical and joined withsharp edges to the bottom wall, even where the latter was highlyreflecting.

The microphotograph of FIG. 8 shows in plan view two, rectangular shapecavities 20-2 a and 20-2 b, bearing on the bottom two metallic coatings5 a and respectively 5 b, consisting of a layer of aluminium. In theright-hand cavity 20-2 a the metallic coating 5 a is away from the sidewall 14 by a distance of a few tens of micron, whereas, in the left-handcavity 20-2 b, the metallic coating 5 b continues under the side wall14. In this microphotograph, the geometric regularity of side wall 14 inboth cavities can be seen, without appreciable differences incorrespondence with the metallic coating 5 b.

EXAMPLE 4

A polymerization inhibitor basic additive consisting oftris-(2-hydroxy-1-propyl)ammina, in a stechiometric ratio of 1:7.5 tothe photoinitiator, was added to the mix obtained following example 2.

The mix was subjected to the process described in example 2, taking careto appropriately increase the energy radiated, taking into account thereduced reactivity of the photopolymerizable layer. The value taken wasof 1.2 J/cm².

Again in this case, the results observed under the SEM, shown in FIG. 9,were excellent. No residues were observed in the masked areas and theside walls of the cavities were vertical and joined with sharp edges tothe bottom wall, even where the latter was highly reflecting.

The microphotograph of FIG. 9 shows, with greater enlargement than theearlier FIGS. 7, 8, a perspective view of almost three quarters of thecavity 20-3 made in example 4, where the production of the two adjacentwalls 14 a and 14 b can be seen clearly, which are joined to the bottomwall 15 with a sharp edge and at right angles. In addition the walls 14a and 14 b show a substantially flat surface, without any significantimperfections, as also does the bottom wall 15.

EXAMPLE 5

A polymerization inhibitor basic additive was added, consisting ofpiperidine, in a stechiometric ratio of 1:5 to the photoinitiator wasadded to the mix obtained following example 2.

The mix was subjected to the process described in example 2, taking careto appropriately increase the energy radiated taking into account thereduced reactivity of the photopolymerizable layer. The value taken wasof 1.8 J/cm².

Observation under the SEM (FIG. 10) revealed that there were no residuesin the masked areas and that the side walls of the cavities werevertical and joined substantially with a sharp edge to the bottom wall,even where the latter was highly reflecting.

The microphotograph of FIG. 10 shows a perspective view of the cavity20-4 made in example 5, where the geometric regularity of the side wallsis apparent and in particular the substantial absence of defects in thearea in which the metallic coating 5 extends under the wall 14 b.

EXAMPLE 6

A polymerization inhibitor basic additive was added, consisting of4-dimethylamminopiridine, in a stechiometric ratio of 1:4 to thephotoinitiator was added to the mix obtained following example 2.

The mix was subjected to the process described in example 2, taking careto appropriately increase the energy radiated taking into account thereduced reactivity of the photopolymerizable layer. The value taken wasof 2.5 J/cm².

Observation under the SEM (FIG. 11) revealed that there were no residuesin the masked areas and that the side walls of the cavities weresubstantially vertical and with a sharp edge to the bottom wall, evenwhere the latter was highly reflecting.

The microphotograph of FIG. 11 shows a perspective view of the cavity20-5, where the production of the two adjacent walls 14 a and 14 b canbe seen clearly, joined to the bottom wall 15 with a sharp edge and atright angles. In addition the walls 14 a and 14 b show a substantiallyflat surface, without any significant imperfections, as also does thebottom wall 15.

The above description relates to a method for producing tridimensionalstructures in a structural layer, obtained from the polymerization ofnegative type photopolymers, which polymerize in the areas struck byradiation and remain soluble, and therefore removable by means of asolvent, in the areas in shadow.

However the method according to this invention also applies to positivetype photopolymers, which, initially polymerized, become soluble in theareas struck by radiation, and therefore removable by means of a solventin the radiated areas. In these photopolymers, if used on substrates ofthe type described earlier, defects could occur, occasioned byreflection phenomena. In the positive photopolymers, in the absence ofthe basic inhibitor compound, the radiation reflected under the maskwould produce an undue solubilization, which would take the form ofundesired voids in the structural layer obtained.

1. A photolithographic process for producing tridimensional structuresin a structural layer in photopolymer for an ink jet printhead, saidtridimensional structures being made on a substrate having a bottom wallat least in part reflecting, and comprising at least a side walladjacent to said bottom wall, said process comprising: depositing onsaid substrate a photopolymerizable layer comprising a solution of amonomer or oligomer, a photoinitiator, and a polymerization inhibitorbasic compound in a quantity less than the stechiometric valuecorresponding to the complete neutralization of the acid speciesgenerated by the photoinitiator; masking said layer for defining areasprotected by said masking and areas not protected by said masking,intended to make said tridimensional structures; irradiating with anultraviolet (UV) radiation for generating acid species suitable forpromoting the polymerization of said solution in said areas notprotected by said masking; in which a fraction of said radiation isreflected by said reflecting part on the inside of said areas protectedby the masking, unwanted acid species being generated in a quantitycorresponding to said fraction of radiation, on account of saidreflected radiation; neutralizing said unwanted acid species by means ofsaid inhibitor basic compound; removing said unpolymerizedphotopolymerizable solution from said areas protected by said masking.2. A process according to claim 1, wherein said polymerization inhibitoris chosen from among: N,N-dimethylaniline; piridine; hydroxylamina;2,6-dimethylpiridine; 4-dimethylamminopiridine; imidazole; morpholine;hydrazine; piperidine; triisopropanolammina; methylamine; ethylamine;diethylamine; triethylamine.
 3. A process according to claim 1, whereinsaid polymerization inhibitor comprises tris-(2-hydroxy-1-propyl)ammina.
 4. A process according to claim 1, wherein said polymerizationinhibitor comprises piperidine.
 5. A process according to claim 1,wherein said polymerization inhibitor comprises4-dimethylamminopiridine.
 6. A process according to claim 1, whereinsaid polymerization inhibitor has a coefficient of acidity of theconjugate acid (pKa) that is between 5 and
 11. 7. A process according toclaim 1, wherein said polymerization inhibitor is present in saidpolymerizable solution in a concentration between 0.05% and 0.4% byweight, with respect to the weight of said polymerizable solution,excluding said solvent.
 8. A process according to claim 1, in which thestechiometric ratio of said polymerization inhibitor to saidphotoinitiator is between 1:10 and 9:10.
 9. A process according to claim1, in which the stechiometric ratio of said polymerization inhibitor tosaid photoinitiator is between 1:8 and 3:8.
 10. A process according toclaim 1, in which said reflecting part consists of a track of aluminium.11. A process according to claim 1, in which said tridimensionalstructures comprise at least one cavity of at least 20 μm side, having abottom wall that comprises a reflecting portion.
 12. A process accordingto claim 1, in which said tridimensional structures have a thickness ofat least 5 μm.
 13. An ink jet printhead produced by a process accordingto claim 1.